WOOD PULPING

Chapter 7

WOOD PULPING 7.1

Background and Definitions

Pulp is the basic product of wood, predominantly used for papermaking, but it is also processed to various cellulose derivatives, such as rayon silk and cellophane. The main purpose of wood pulping is to liberate the fibers, which can be accomplished either chemically or mechanically or by combining these two types of treatments. The common commercial pulps can be grouped into chemical, semichemical, chemimechanical, and mechanical types (Table 7-Ί). The term "high-yield pulp" is often collectively used for different types of lignin-rich pulps needing mechanical defibration. Chemical pulping is a process in which lignin is removed so completely that the wood fibers are easily liberated on discharge from the digester or at most after a mild mechanical treatment. Practically all of the production of chemical pulps in the world today is still based on the sulfite and sulfate (kraft) processes, of which the latter predominates. This chapter deals main­ ly with these pulping processes. Sulfite Pulping The first patent dealing with pulping of wood with aque­ ous solutions of calcium hydrogen sulfite and sulfur dioxide in pressurized systems was granted in 1866. This pioneering invention, made in the United

114

7.1 Background and Definitions TABLE 7 - 1 .

115

Commercial Pulp Types Pulp type a

A. Chemical Acid sulfite Bisulfite Multistage sulfite Anthraquinone alkali sulfite Kraft Polysulfide-kraft Prehydrolysis-kraft Soda B. Semichemical NSSC Green liquor Soda C. Chemimechanical Chemithermomechanical (CTMP) Chemigroundwood (CGW) D. Mechanical Stone groundwood (SGW) Pressure groundwood (PGW) Refiner mechanical (RMP) Thermomechanical (TMP)

Yield (% of wood) 35-65

70-85

85-95

93-97

d Main uses: Type A: various paper qualities, board, liner, cellulose derivatives. Type B: board, liner, corrugating medium. Type C: tissue, fluff, etc. Type D: newsprint, supercalendered (SC) paper, and lightweight coated (LWC) paper.

States by B. Tilghman, can be considered to be the origin of the sulfite pulping process. It required almost one decade before the world's first sulfite pulp mill started its production in Sweden in 1874. This was accomplished by C. D. Ekman, who is the principal initiator of the sulfite pulp industry. Essentially, sulfite pulping is still based on these old inventions, although several innovative modifications and technical improvements have been introduced. The later achievements during the 1950s and the 1960s con­ cerned the introduction of so-called soluble bases, that is, replacement of calcium by magnesium, sodium, or ammonium, giving much more flexibil­ ity in adjusting the cooking conditions, extending both the raw material basis and production of different pulp types. Also, methods for the recovery of these bases as well as sulfur were developed. Although until the 1950s most of the pulp in the world was based on the sulfite process, the kraft process has gradually taken over its dominating position. However, the

116

Chapter 7. Wood Pulping

sulfite process is still important at least in certain countries and for some pulp qualities. Kraft Pulping Pressurized alkaline cooking systems at high temperatures were introduced in the 1850s. According to the method proposed by C. Watt and H. Burgess, sodium hydroxide solution was used as a cooking liquor and the resulting spent liquor was concentrated by evaporation and burned. The smelt, consisting of sodium carbonate, was reconverted to sodium hydroxide by calcium hydroxide (caustisizing). Since sodium car­ bonate was used for makeup, the cooking process was named the soda process. In 1870, A. K. Eaton in the United States patented the use of sodium sulfate instead of sodium carbonate. Similar ideas were pursued by C. F. Dahl, who about 15 years later presented a technically feasible pulping process in Danzig. These inventions initiated the sulfate {kraft) process. However, the breakthrough of the kraft process came first in the 1930s after introduction of multistage bleaching systems. Most important was the pi­ oneering work by G. H. Tomlinson in Canada, who developed a recovery furnace suitable for combustion of the kraft "black" liquors. In the kraft process sodium sulfate is added for makeup. It is reduced in the recovery furnace to sodium sulfide, which is the key chemical needed for delignification. The kraft process has almost completely replaced the old soda process because of its superior delignification selectivity resulting also in a higher pulp quality. Since the 1960s the production of kraft pulps has also in­ creased much more rapidly than that of sulfite pulps because of several factors, such as a simpler and more economic recovery of chemicals and better pulp properties in relation to market needs. The introduction of effec­ tive bleaching agents, especially chlorine dioxide, has eliminated the earlier difficulties involved in the bleaching of kraft pulps to a high brightness, and prehydrolysis of wood has made it possible to produce high-grade dissolving pulps by the kraft process. Although today more than 80% of the chemical pulp produced in the world is kraft pulp, the kraft process still suffers from several weak sides which are difficult to master. These are the malodorous gases and the high consumption of bleaching chemicals of softwood kraft pulps. However, ac­ cording to recent progress it is to be expected that new modifications will lead to improvements with regard to the environmental needs. High-Yield Pulping Cooking of wood chips at less drastic conditions or shorter time, leading only to a partial dissolution of lignin, results in semichemical pulps. The chemical reactions are similar to those for chemical pulping at corresponding conditions. Neutral sulfite semichemical (NSSC)

7.1 Background and Definitions

117

pulps represent a usual type and they are produced by cooking the chips with sodium sulfite-bisulfite solutions prior to the mechanical defibration in disk refiners. The fiber properties of hardwood NSSC pulps make them suitable especially for corrugating medium. Chemimechanical pulps (CMP) are produced by pretreating the wood chips before defibration at rather mild conditions, usually with alkaline solutions of sodium sulfite at elevated temperatures. By this treatment sulfonie acid groups are introduced into the lignin, making it more hydrophilic and increasing the degree of swelling of the pulp. In this category so-called chemithermomechanical pulps (CTMP) are rather recent newcomers, and the first mill producing this type of softwood pulp started in 1979 in Sweden. Because of the chemical pretreatment the physical fiber damage in the final mechanical defibration stage is less severe and the strength properties of the pulps obtained are better than those of the thermomechanical pulps (TMP) (see later). Although the energy demand is also lower, the process leads to an increased dissolution of the wood substance, which means special pro­ cedures for handling and purification of the dilute process solutions in order to avoid pollution of recipients. Mechanical Pulping The process of wood grinding in which barked wood logs are treated in a rotating grindstone in the presence of water spray forms the basis for mechanical pulping. In addition to whole fibers, the wood substance is torn off in the form of more or less damaged fiber frag­ ments. This physical fiber damage cannot be avoided and the strength of the paper made from mechanical pulps is thus rather low. Additional drawbacks of mechanical pulping are the high energy demand and that practically only softwoods, mainly spruce, are useful raw materials. The method of producing stone groundwood pulp (GW or SGW) was developed around 1840 by F. G. Keller in Germany. Newer development during the 1970s resulted in a modified groundwood process in which grinding is done at higher pressures. Since the temperature in the grinding stone is higher, lignin is softened, which favors defibration. Consequently, the pressure groundwood pulp (PGW) has somewhat better strength proper­ ties than the conventional GW pulp. Another technique of mechanical defibration of wood is to use disk refin­ ers, which, of course, requires a preceding chipping. An improved defibra­ tion technology was developed in the 1960s resulting in so-called ther­ momechanical pulps (TMP). This type of mechanical pulping means refining after pressurized presteaming and it results in improved strength properties of the pulp. However, a disadvantage is a high energy demand. Unconventional Pulping During recent years increasing attention in the pulp industry has been focused on minimizing pollution and saving energy.

118

Chapter 7. Wood Pulping

In the sulfite pulp industry a complete recovery of sulfur dioxide from the flue gases is difficult and costly, but such emissions cannot be accepted because they cause serious environmental effects. In the kraft process this type of emissions does not cause problems, but instead an inherent disad­ vantage is the generation of volatile organic sulfur compounds, which, even when present as traces, emit an unpleasant odor to the surroundings. This is the reason why kraft pulp mills cannot be established near more crowded residential areas and have not been accepted in countries like West Ger­ many and Switzerland. It is therefore understandable why alternative, "un­ conventional" processes and possibilities for sulfur-free pulping have at­ tained much interest. In the middle 1970s it was found that anthraquinone is an effective delignification agent, improving delignification markedly in alkaline conditions like hydrogen sulfide ions. This was a remarkable discovery particularly because only small amounts of anthraquinone are needed to accelerate the delignification markedly. Both anthraquinone and tetrahydroanthraquinone are important delignification agents, but they are today mostly used only as additives for kraft and alkaline sulfite pulping. Pulping in the presence of organic solvents represents a more radical approach. Although the idea came up as early as the beginning of 1930s, it did not attain serious attention until rather recently. The laboratory and pilot plant experiments so far performed include a number of solvents and cata­ lysts in both acidic and alkaline conditions, but so far at least "organosolv" pulping of softwood seems not very promising and the solvent recovery is problematic. It can therefore be doubted whether organosolv pulping pro­ cesses can be competitive enough to be introduced more universally into large-scale operations. Oxygen is an attractive delignification agent. It is today successfully ap­ plied for pulp bleaching, but practically no useful oxygen pulping processes have emerged from the intensive research. Application of lignin-degradative microorganisms as delignification agents has also attained much interest. The idea is not new, but the "blopulping" processes are still unrealistic despite of the rapid development of bio­ technology. So-called steam explosion processes finally represents an unusual type of pulping in which the wood is subjected to a short treatment at high tem­ peratures (200°-250°C) and pressures, followed by a release to atmospheric pressure. After this type of treatment wood is disintegrated, but the fiber structure is extensively damaged. The potential uses for products obtained after steam explosion may include ruminant feeding, fermentation sub­ strates, and chemical feedstocks.

7.2 Sulfite Pulping

7.2

Sulfite Pulping

7.2.1

Cooking Chemicals and Equilibria

119

Sulfur dioxide is a two-basic acid and the following equilibria prevail in its aqueous solution: SO., + H 2 0 ^ H,S03 (SO,·H 2 0)

(7-1 )

H,S03 ?± H + + HSO3-

(7-2)

2

HSO3- +± H + + SO3 -

(7-3)

Since the concentrations of sulfur dioxide in its free (S0 2 ) and hydrated ( S 0 2 - H 2 0 or H 2 S0 3 ) forms cannot be determined separately, equations (7-1) and (7-2) are combined to give expression (7-4) in which the total sulfur dioxide concentration is in the denominator. This defines the first equilibrium constant ΚΛ: Kx = [H+] [HS0 3 -]/([S0 2 ] + [H2S03])

(7-4)

The second equilibrium constant derived from equation (7-3) is K2 = [H+] [SO32-] / [HSO3-]

(7-5)

By taking the logarithms of both sides of equations 7-4 and 7-5, the follow­ ing expressions are obtained: pK, = pH - log{[HS03-] / ([S02] + [H 2 S0 3 ])} = 2

(7-6)

pK2 = pH + logüHSCVl / [SO32-]) = 7

(7-7)

It should be noted that Κλ and K2 are not thermodynamic constants since the activity coefficients have been neglected, and hence they are strictly valid only at a given concentration. It follows from these equilibria that the relative concentrations of sulfur dioxide, hydrogen sulfite, and sulfite are governed by the pH of the solution (Fig. 7-1 ). As can be seen, sulfur dioxide is present almost exclusively in the form of hydrogen sulfite ions at pH around 4. Below and above this value the concentrations of sulfur dioxide and sulfite ions, respectively, are suc­ cessively increased. These equilibria also vary with the temperature. At temperatures used for pulping (130°-170°C), the actual pH value is higher than that measured at room temperature and this deviation is larger in the acidic region. Because of the low solubility of calcium sulfite, a large excess of sulfur dioxide is required to avoid its formation from calcium hydrogen sulfite.

120

Chapter 7. Wood Pulpir»g 100

/

90 80 i m

/ - I

X ^

50

1

40 -1

E

30 20

o

\\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ n \\ \\

/

70

10

"1

,

1

1

2

1

L

3

4

1

5

1

6

1

7

l'^'^ri

8

9

10

PH Fig. 7-1. The molar ratio of hydrogen sulfite ion concentration to total sulfur dioxide as a function of pH at 25°C (Sjöström et al., 1962). —, 10 g Na 2 0/liter; —, 50 g Na20/liter. ρΚλ ~ 1.7; pK2 ~ 6.6-6.8.

Calcium base is thus usable only for acid sulfite pulping. Magnesium sulfite is much more soluble. When using magnesium as base, the pH can be increased to about 4 - 5 , but above this range magnesium sulfite starts to precipitate and in alkaline region magnesium precipitates as hydroxide. Sodium and ammonium sulfites and hydroxides are easily soluble, and the use of these bases have no limitations in the pH of the cooking liquor. There are several modifications of the sulfite method which are designated according to the pH of the cooking liquor (Table 7-2). For the production of chemical pulps, delignification is allowed to proceed until most of the lignin in the middle lamella is removed after which the fibers can be readily separated from each other. Semichemical pulps are often produced by the neutral sodium sulfite method followed by mechanical fiberization of the partially delignified wood. According to the usual but rather misleading convention, the total amount of sulfur dioxide is divided into "free" and "combined" sulfur dioxide. For example, sodium hydrogen sulfite solution contains equal amounts of com­ bined and free sulfur dioxide (2 NaHS0 3 - + Na 2 S0 3 + S 0 2 + H 2 0), although essentially no free sulfur dioxide exists in such a solution. The term "active base" refers to the sum of the hydrogen sulfite and sulfite ions and is usually expressed as oxide, e.g., CaO or N a 2 0 . A typical acid sulfite cook­ ing liquor contains about 10 g and 60 g combined and free sulfur dioxide per liter, respectively.

TABLE 7-2.

Sulfite Pulping Methods and Conditions

Method Acid (bi)sulfite Bisulfite

pH range

"Base" alternatives

Active reagents

Max. temp. (°C)

Time at max. temp. (hr)

Softwood pulp yield

(%)

1-2

Ca 2 + , M g 2 + ,

HSO3-, H +

125-145

3-7

45-55

3-5

Na + , N H 4 + M g 2 \ Na + ,

HSO3-, H +

150-170

1-3

50-65

50-60

NH 4 + Two-stage sulfite (Stora type) Stage 1 Stage 2 Three-stage sulfite (Sivola type) Stage 1 Stage 2 Stage 3 Neutral sulfite (NSSC) Alkaline sulfite" α b

6-8 1-2

Na+ Na+

HSOa", SO32HSO3-, H+

135-145 125-140

2-6 2Λ

6-8 1-2 6-10 6-9 9-13

Na + Na + Na +

HSO3-, SO32HSO3-, H+ HOHS03-, S032" S032-, H O -

120-140 135-145 160-180 160-180 160-180

2-3 3-5 2-3 0.25-3 3-5

N a \ NH 4 + Na +

Hardwood. Including this method in the presence of anthraquinone (AQ).

35^5 75-90° 45-60

122

7.2.2

Chapter 7. Wood Pulping

Impregnation

The cooking process begins with an impregnation stage after the chips have been immersed in the cooking liquor. This stage involves both the liquid penetration into wood cavities and the diffusion of dissolved cooking chemicals. The rate of penetration depends on the pressure gradient and proceeds comparatively rapidly, whereas diffusion is controlled by the con­ centration of dissolved chemicals and takes place more slowly. Penetration is influenced both by the pore size distribution and capillary forces while diffusion is regulated only by the total cross-sectional area of accessible pores. A good impregnation is a prerequisite for a satisfactory cook. If the trans­ port of chemicals into the chips is still incomplete after the cooking tem­ perature has been reached, undesirable reactions catalyzed by hydrogen ions will occur. For instance, if the base concentration in an acid sulfite cook is insufficient the sulfonic acids formed are not neutralized, and the pH value of the cooking liquor drops sharply. Because of the low pH value, reactions leading to lignin condensation as well as to decomposition of the cooking acid are accelerated in the interior of the chips resulting in dark, hard cores. In the choice of the length, pressure and temperature of impreg­ nation, consideration must be given to the wood used; for example, heartwood is much more difficult to impregnate than sapwood. 7.2.3

Morphological Factors

Ideally, the purpose of pulping is to remove the lignin as completely as possible and preferably from the middle lamella. In reality, however, the KRAFT

Q UJ

> o Σ

LU

ACID SULFITE

^-\

0 Ρ«,,.^^^ 20 ■ \

>v

- s\

40

ML

\ \

60

z o

>

80 1

0

L

^Λχ

\

1

1

1

20 40 60 80

1

0

1

1

1

20 40 60 80

DELIGNIFICATION OF WOOD (°/·) Fig. 7-2. Delignification of the secondary wall (S) and compound middle lamella (ML) during kraft and acid sulfite pulping (Wood and Goring, 1973). Note that the S wall is delignified faster than the ML layer at the earlier stages of the cook.

7.2 Sulfite Pulping

123

KAPPA NUMBER

0.6

u z <

CD CL

O m

<

KRAFT

O.ül·

0.2

4

8

8

16

LIGNIN CONTENT (%> of pulp) Fig. 7-3. UV absorbance (222 nm, 0.5 μηι section thickness) by various morphological re­ gions of spruce fibers delignified to various lignin contents by the kraft and acid sulfite method (Wood and Goring, 1973). S, secondary wall; P, primary wall; CCP, primary wall at the cell corner.

polysaccharides located mainly in the secondary wall region are attacked by the cooking chemicals and their losses cannot be avoided. Despite of conflicting opinions it is to be expected that the chemicals diffuse gradually from lumen through the cell layers reaching the middle lamella at the end. Indeed, this view has been supported by experimental data according to which the delignification during both kraft and sulfite pulping proceeds faster in the secondary wall than in the middle lamella (Fig. 7-2). Toward the end of the cook the differences in lignin concentration across the cell walls are reduced to a more uniform level (Fig. 7-3). Most of the lignin in the final pulp fibers is located in the secondary wall and in the cell corners (Table 7-3). TABLE 7-3.

Distribution of Lignin in Kraft and Acid Sulfite Fibers of Spruce Early wood J Proportion of total lignin in fiber (%)

α

Acid sulfite

Kraft

Pulping method: Kappa number:

50

25

50

25

15

Secondary wall Primary wall Cell corner

73 14 13

87 10 3

88 8 4

90 8 2

92 8 0

From Wood and Goring (1973).

124

7.2.4

Chapter 7. Wood Pulping

General Aspects of Delignification

Simultaneously with the dissolution of lignin, more or less carbohydrates are removed from the wood during pulping. The selectivity of delignification can be expressed as the weight ratio of the lignin and carbohydrates re­ moved from the wood after a certain cooking time or at a given degree of delignification. A high selectivity thus means low carbohydrate losses. Fig­ ure 7-4 illustrates this situation for various pulping methods. The losses of carbohydrates are high in the beginning of the cook, which means that they are attacked even at a relatively low temperature when delignification still proceeds slowly. After an improved middle period of delignification, a rather abrupt change in the selectivity takes place toward the end of the cook. This is the point when the cook should be interrupted in order to avoid high yield losses and impairment of pulp properties. Basically, two types of reactions, sulfonation and hydrolysis, are responsi­ ble for delignification in sulfite pulping. Sulfonation generates hydrophilic sulfonic acid groups in the hydrophobic lignin polymer, while hydrolysis breaks ether bonds between the phenylpropane units, lowering the mo­ lecular weight and creating free phenolic hydroxyl groups. Both of these

LIGNIN REMOVED (% OF WOOD) Fig. 7-4.

Comparison of delignification selectivities.

7.2 Sulfite Pulping

0.5 0.4

125

pH 2.5, 2.5% S 0 2 9.0% S 0 2 s pH 3.5, 2.5% S 0 2 5.0% SO*

co

o o

0.3 Ζ

0.2 pH 5-9, 5% SO

0.1 0

J-

_L

-L

12

16

18

2

A

ι y 20

24

TIME (hours) HC—OH

OCH*

HC—O—C

OCH<

HC—OH

OCHÎ

HC—O—C

OCH3

Fig. 7-5. The degree of sulfonation of lignin (measured as S/OCH 3 ) after treatments of soft­ wood meal (spruce) at various pH conditions (135°C). The structures within group A are sulfonated already at a relatively neutral pH region and the rate of reaction of the X types is high. Those within group B are sulfonated only at acidic conditions. This figure roughly illus­ trates the sulfonation of the benzyl position in free and etherified lignin structures. (Adopted from Lindgren, 1952.)

reactions increase the hydrophilicity of the lignin, rendering it more soluble. At the pH conditions used for neutral and alkaline sulfite pulping, the hydrolysis reactions are very slow compared with sulfonation, and the de­ gree of sulfonation of lignin remains low (Fig. 7-5). Delignification therefore proceeds slowly. At the conditions of acid sulfite pulping hydrolysis is fast compared with sulfonation, which assumes the role of rate-determining step. Because lignin is also sulfonated to a fairly high degree, the conditions are favorable, promoting extensive dissolution of lignin. A certain amount of base is required for neutralization of both the lignosulfonic acids and the acidic degradation products of the wood substance

126

Chapter 7. Wood Pulping 12 11

M OIUV.K

IUURS

o Normal or brownish cooks

10

^

9

o

£

8

"> 6 _J

<

5 3 2 1 0.25

0.50

0.75

1.00

1.25

COMBINED 50 2 (%») Fig. 7-6. Influence of cooking acid composition on lignin condensation in acid sulfite cooking of spruce (130°C) (Kaufmann, 1951).

formed in the side reactions. If the buffering capacity is insufficient, the pH is sharply decreased and the rate of competing condensation reactions is in­ creased. These harmful reactions result in a decreased delignification or prevent it completely (Fig. 7-6). 7.2.5

Lignin Reactions

At a given temperature, the extent of delignification depends largely on the acidity of the cooking liquor. Conditions typical of acid sulfite pulping (140°C, pH 1-2) result in effective delignification, whereas after a corre­ sponding treatment in neutral sulfite solution most of the lignin remains insoluble. The average degree of sulfonation of the undissolved softwood lignin, expressed as the molar ratio of sulfonic acid groups to lignin methoxyl, also remains low ( S 0 3 H / O C H 3 ~ 0.3). In contrast, the degree of sulfonation of softwood lignosulfonates dissolved during acid sulfite pulping is much higher or about 0.5 (cf. Fig. 7-5). This type of difference is also typical for hardwood lignin, but the degree of sulfonation is throughout much lower than that of softwood lignin. Most of the sulfonic acid groups introduced into the lignin replace hydroxyl or ether functions at the a-carbon atom of the propane side chain. The sulfonation proceeds rapidly at all pH values when the phenolic hydroxyl group located at the para position is free. Experiments especially with dimeric model compounds representing various structures and bond types

7.2 Sulfite Pulping

127

OCH, "3

A HC©

"

OCH3 .0 IOH) 2a S02-H20 a - H® OCH3

HC-O-^ 7

V

I

HC

0CH 3

Fig. 7-7. Behavior of ß-aryl ether and open α-ether structures during acid sulfite pulping (Gellerstedt and Gierer, 1971). R = H, alkyl, or aryl group. The first reaction step involves cleavage of the a-ether bond with formation of a resonance-stabilized carbonium ion which is then sulfonated. Note that both the phenolic and nonphenolic structures are sulfonated, while the ß-aryl ether bonds are stable.

in lignin have provided insight into these reactions. Under acidic conditions the most important structural units in lignin are sulfonated irrespective of whether they are free or etherified. Under neutral conditions, however, sulfonation as well as cleavage of the ether bonds, leading to lignin fragmen­ tation, is essentially restricted to the phenolic units. CHO I

HCOH II CH

CHO I CH II HC

HOHCSO® I 3

CH5

CH9

1 θ HCSof

HC®

' Θ HCS0°

^Γ *KT
»

CH20H °ξ*

®CH2

A.

-'

°f*

H2CSO? « ¥ *

fr0HW^

HÌ-OHQK C=0

θ

_

COH

/

HÌ-0-f\ Λ

C=0

HSpy_ -H 2 0 OCH3

Fig. 7-8. Sulfonation of coniferaldehyde end groups and substituted structures containing acarbonyl groups (see Gellerstedt, 1976). At lower temperatures aldehyde end groups can bind sulfur dioxide because of the formation of a-hydroxysulfonic acid.

128

Chapter 7. Wood Pulping

Acid Sulfite (and Bisulfite) Pulping During acid sulfite pulping the ahydroxyl and the a-ether groups are cleaved readily under simultaneous formation of benzylium ions (Fig. 7-7). This reaction takes place regardless of whether the phenolic hydroxyls of the phenylpropane units are etherified or free. The cleavage of open a-aryl ether bonds represents the only note­ worthy fragmentation of lignin during acid sulfite pulping. Although rela­ tively few open a-aryl ether bonds are present in softwood lignin, their cleavage results in a considerable fragmentation. The benzylium ions are sulfonated by attack of hydrated sulfur dioxide or bisulfite ions present in the cooking liquor. The coniferaldehyde end groups and ß-substituted structures containing a-carbonyl groups are also sulfonated (Fig. 7-8). The benzylium ions formed from the 1,2-diarylpropane structures are easily converted to stilbene structures by elimination of a hydrogen ion at the ß-position, after which the electrophilic 7-carbon atoms can be sulfonated (Fig. 7-9). Condensation reactions of carbonium ions compete with sulfonation and their frequency is increased with increasing acidity. Carbon-carbon bonds are formed most commonly when the benzylium ions react with the weakly nucleophilic 1- and 6-(or 5-)positions of other phenylpropane units (Fig. 7-10). Subsequently, propane side chains and hydrogen ions are eliminated, CH20H

/

0 C H3

©CH 2

i klì II y^ocH 3

CH H® -H 20

l^iì A

H2CS0?

3

\

P C H3

c^fV-o II \ = / \ CH

HSO©

I

(ίχ\

"S 0CH 3

^

,0

Fig. 7-9.

CH

ζII - τ\ \= / - 0

{11 - /\=/V u \ CH

/°

L A 0CH 1 3 "S /0

Reactions of stilbene structures during acid sulfite pulping (see Gellerstedt, 1976).

OCH3 HC®

' y^ocH3 ^ 0 (OHI

HC-O-H

' XT"OCH; 0®l®0Hl

"

HCO I

Fig. 7-10. Examples of lignin condensation products formed during acid sulfite pulping (Gierer, 1970). Condensation results from the reaction of a carborrium ion with the weakly nucleophilic sites in the benzene nucleus.

7.2 Sulfite Pulping

129

OH ^OCH,

H2C

"CH

HC

CH

I

H

I

I

HOH 2C v

I

9^0^

C H

HOH2

H®,HS0 3e|

2

ff I

I C

^r

ιί

IIN

^

^0CH3

^OH

rìì

H 3C O '

JyÌ

Fig. 7-11. Intramolecular condensation of pinoresinol structures during acid sulfite pulping (see Gellerstedt, 1976).

respectively. In general, the condensation reactions result in increased mo­ lecular weight of the lignosulfonates and the solubilization of lignin is re­ tarded or inhibited. However, the benzylium ions formed from phenyl coumaran and pinoresinol structures can be condensed intramolecularly with­ out increasing the molecular weight (cf. Fig. 7-11). During acid sulfite pulping, lignin may also condense with reactive phe­ nolic extractives. Pinosylvin and its monomethyl ether, present in pine heartwood, are examples of phenolic extractives of this type. Dual conden­ sation of pinosylvin with lignin generates harmful cross-links. Consequently, pine heartwood cannot be delignified by the conventional acid sulfite method. Cross-links between lignin entities may also be generated by thiosulfate present in the cooking liquor (Fig. 7-12). This results in retarded delignification and, under certain circumstances, in complete inhibition ("black cook"). For the formation of thiosulfate, see Section 7.2.8. Neutral and Alkaline Sulfite Pulping In neutral sulfite pulping the most important reactions of lignin are restricted to phenolic lignin units only. The first stage always proceeds via the formation of a quinone methide with

çr 00 " 3 .

—

HS2of

-ç-s-so3e Fig. 7-12.

!J

♦

H20

-c1

OH φ Γ ΟΟΗ 3

CÎ-s-sof 1

-C-OH

* HO-

OH

OH OH iyOCH3 ^γΟΟΗ3

T

T -c — s

cI

HSO? 1

T c1

Reactions of lignin with thiosulfate (see Goliath and Lindgren, 1961).

130

Chapter 7. Wood Pulping

Fig. 7-13. Reactions of phenolic ß-aryl ether and a-ether structures (1) during neutral sulfite pulping (Gierer, 1970). R = H, alkyl, or aryl group. The quinone methide intermediate (2) is sulfonated to structure (3). The negative charge of the a-sulfonic acid group facilitates the nucleophilic attack of the sulfite ion, resulting in ß-aryl ether bond cleavage and sulfonation. Structure (4) reacts further with elimination of the sulfonic acid group from a-position to form intermediate (5) which finally after abstraction of proton from ß-position is stabilized to a styrene-ß-sulfonic acid structure (6). Note that only the free phenolic structures are cleaved, whereas the nonphenolic units remain essentially unaffected.

simultaneous cleavage of an a-hydroxyl or an a-ether group (Fig. 7-Ί3). At least in noncyclic structures, the quinone methide is readily attacked by a sulfite or a bisulfite ion. The a-sulfonic acid group formed facilitates the nucleophilic displacement of the ß-substituent in ß-aryl ether structures by a sulfite or bisulfite ion. Subsequent loss of the a-sulfonate group leads to a styrene-ß-sulfonic acid structure, especially at higher pH values (>7). The

CH20H

H3co

H3ccr " y ^ç

H3CO

OH

OCH3

R is H or CH 20H

R is H or CH2SO3

Fig. 7-14. Reaction of phenyl coumaran structures during neutral sulfite pulping (see Gellerstedt, 1976).

7.2 Sulfite Pulping CH2OH °vCH3

:-0-^VHC-O-i' >i c=o

V=/ /

131

HCSof

H2CSO? H2CSO3 i c=o

^λ

+

CH CH i c=o

Cl -°

Fig. 7-15. Cleavage of ß-aryl ether bonds in structures containing α-carbonyl groups (see Gellerstedt, 1976). Note that this reaction can take place even in nonphenolic units.

cleavage of the a- as well as the ß-aryl ether bonds naturally generates new reactive phenolic units. The quinone methides can also react by elimination of formaldehyde or hydrogen ion at the ß-carbon atom, especially when the formation of conju­ gated diaryl structures is possible. Examples of this type of reactions are the formation of stilbenes from phenyl coumarans or 1,2-diarylpropane struc­ tures and that of 1,4-diarylbutadienes from pinoresinol structures (cf. Fig. 7-14). The conjugated structures can be further sulfonated at their a-carbon atoms. Moreover, the quinone methides can condense with nucleophilic sites of other phenylpropane units or with thiosulfate. The condensation products are similar to those formed during acid sulfite pulping. Carbonyl groups present in lignin may have a great influence on its reac­ tions with neutral sulfite. For example, a-carbonyl groups can activate the ßaryl ether bonds in nonphenolic units and induce their cleavage (Fig. 7-15). Coniferaldehyde end groups are also extensively sulfonated. Finally, methoxyl groups which are completely stable toward acid sulfite may, in part, be cleaved during neutral sulfite pulping with the formation of methane sulfonic acid (Fig. 7-16). Although adequate information is lacking, the reactions of lignin with alkaline sulfite are largely related to those occurring in neutral sulfite and alkali pulping. During alkaline sulfite pulping the ß-aryl ether bonds are obviously cleaved also in nonphenolic units, and the condensation reac­ tions have been proposed to be less important compared with those during kraft pulping.

CHaSOa0 ^ΟΙΟΗ)

^ΟΙΟΗ]

Fig. 7-16. Cleavage of the methyl aryl ether bond with formation of methanesulfonic acid during neutral sulfite pulping (Gierer, 1970).

132

7.2.6

Chapter 7. Wood Pulping

Carbohydrate Reactions

The mechanisms of the reactions of carbohydrates have been dealt with in Sections 2.5.4 and 2.5.5. In this section consideration is given to the changes of wood polysaccharides during pulping as well as to the reaction products and pulp yield. Acid Sulfite Pulping Because of the sensitivity of glycosidic linkages toward acidic hydrolysis, depolymerization of wood polysaccharides cannot be avoided during acid sulfite pulping. Hemicelluloses are attacked more readily than cellulose due to their amorphous state and a relatively low degree of polymerization. Moreover, most of the glycosidic linkages of hemicelluloses are more labile toward acid hydrolysis than those of cel­ lulose. When the hydrolysis has proceeded far enough, the depolymerized hemicellulose fragments are dissolved in the cooking liquor and are gradu­ ally hydrolyzed to monosaccharides. The ordered structure protects cellulose. Only a moderate depolymeriza­ tion of cellulose and practically no yield losses take place unless the delignification is extended to very low lignin contents and the conditions are rather drastic as is the case when producing dissolving pulp. After a certain degree of depolymerization the fiber strength weakens drastically, which, of course, is not acceptable in the case of paper-grade pulps. The main hemicellulose component of softwoods constitutes of acetylated galactoglucomannans. The galactosidic linkages are completely hydrolyzed at normal sulfite pulping conditions. These conditions are also strong enough to remove the acetyl groups. Glucomannan is thus the component remaining in the pulp. Arabinoglucuronoxylan, the other hemicellulose constituent of softwoods, is converted to glucuronoxylan. Because the furanosidic linkages of the arabinose units are extremely labile toward acid, they are cleaved already at early stages of the cook. The glycuronide bonds are exceptionally stable toward acid, contrary to the glycosidic bonds. However, the glucuronic acid content of the xylan fraction remaining in pulp is lower that of the native xylan. The xylan frac­ tions heavily substituted with high uronic acid groups are obviously more readily dissolved, whereas those having fewer side chains are preferentially retained in the pulp. Hardwood hemicelluloses are mainly composed of acetylated glucu­ ronoxylan, which is also extensively deacetylated during pulping. The frac­ tions of low glucuronic acid contents are preferentially retained in the pulp. The polysaccharides present in both softwoods and hardwoods in minor quantities, such as starch and pectins, are dissolved already at early stages of

7.2 Sulfite Pulping

133

the cook. The chemical pulp is practically free from these polysaccharide components. The hydrolyzed and soluble sugar fragments are not completely stable at the cooking conditions. In addition to various minor degradation products, about 1 0 - 2 0 % of the monosaccharides are oxidized to aldonic acids by the hydrogen sulfite ions. This is an important reaction particularly because it gives rise to the formation of thiosulfate: 2R-CHO + 2HS03- - * 2R-C02H + S 2 0 3 2 - + H 2 0

(7-8)

In addition to the formation of aldonic acids, a minor fraction of the mono­ saccharides is converted to sugar sulfonic acids. Generally no cellulose is lost in the acid sulfite process. The hemicellulose yield losses are higher for hardwood than for softwood. Typical material balances resulting from acid sulfite (paper-grade) pulping of both softwood (spruce) and hardwood (birch) are given in Table 7-4. For com­ parison, the corresponding values for kraft pulping of pine and birch are included in this table. Multistage Sulfite Pulping The carbohydrate yield can be improved con­ siderably by applying a two-stage pulping method. According to the "Stora method" the wood chips are first precooked in a sodium bisulfite-sulfite solution, usually at pH of 6 - 7 , and then subjected to a second cooking stage at acid sulfite pulping conditions. In the first stage lignin is sulfonated to a TABLE 7-4. Yields of Various Pulp Constituents after Sulfite and Kraft Pulping of Norway Spruce (Picea abies), Scots Pine (Pinus sylvestris), and Birch (Betula verrucosa)3

Cellulose Glucomannan Xylan Other carbohydrates and various components Sum of carbohydrates Lignin Extractives Sum of components (total yield) α b

Birch sulfite

Spruce sui If ite

Constituents 41 5 4

— 50 2 0.5 52

(41)ft (18) (8) (4) (69) (27) (2) (100)

40 1 5

—

(40) (3) (30) (4)

46 2 1 49

(74) (20) (3) (100)

The figures are calculated as percent of the dry wood. Figures in parentheses refer to the original wood composition.

Pine kraft 35 4 5

— 44 3 0.5 47

Birch kraft (39) (17) (8) (5)

(67) (27) (4) (100)

34 1 16 — 51 2 0.5 53

(40) (3) (30) (4) (74) (20) (3) (100)

134

Chapter 7. Wood Pulping

certain degree but is mainly retained in the solid wood phase. Delignification is accomplished in the second, acidic cooking stage. Compared to conventional acid sulfite pulping the two-stage process results in an appre­ ciably higher pulp (carbohydrate) yield. This yield increase strongly depends on the pH in the first stage and is maximally about 8%, calculated on the wood basis. Because the pulp properties are rather strongly dependent on the hemicellulose content of the pulp, full advantage from the higher yield can be taken only when the pulp is applied for certain purposes, such as for making transparent (glassine) papers. However, by adjusting the pH in the first stage, the yield can be varied so that optimum pulp properties are obtained for different application purposes. An additional and perhaps the main advantage of the two-stage pulping process is the fact that pine heartwood can be processed, which is not possible when applying the conventional acid sulfite process. In the first stage the reactive groups of lignin are protected by sulfonation, which blocks their condensation reactions with phenolic extractives (pinosylvin and taxifolin). The yield increase in two-stage pulping is mainly restricted to softwoods and associated with an increased retention of glucomannans in the pulp (Fig. 7-17). This, in turn, is a consequence of their deacetylation taking place in the first cooking stage. There is a close relationship between the acetyl content of the precooked chips (which can be regulated by adjusting the pH) and the final pulp (or glucomannan) yield. The corresponding ester linkages are extremely sensitive toward alkaline hydrolysis and they are cleaved completely when applying neutral cooking liquors at a normal cooking temperature, provided that the cooking time is sufficiently long. It should be noted that at higher temperatures the equilibrium of reaction (7-1 ) (p. 119) is shifted to the right and the ion product of water is increased. As a result of these two combined effects the pH of the cooking liquor is higher at the cooking temperature than at room temperature. A probable explanation for the behavior of deacetylated glucomannans is that they are more closely hydrogen bonded to the cellulose microfibrils, or partly crystallized, in which state their resistance toward acid hydrolysis in the second pulping stage is improved. By contrast, two-stage pulping of hardwoods only moderately improves the carbohydrate (xylan) yield in comparison with conventional pulping. The glucuronic acid substituents possibly counteract association of xylan with cellulose. Two-stage sulfite pulping of hardwood is not applied in the pulp industry. Other modifications of two-stage (or multistage) sulfite pulping are also used commercially. The "Sivola method" is suitable for producing dissolving pulps with a high cellulose content. After delignification with acid sulfite the

7.2 Sulfite Pulping

135

SOLID PHASE (% OF WOOD) ST

AFTER 1 STAGE

100 r EXTRACTIVES. E T C . = E

AFTER 2 ^ TÎJAQE /

Γ

pH4

pH4*

pH7*

90

pH1.5

PH1.5

80

OTHER

GA=3

CARBOHYDRATES=GA

(ARABINO)GLUCURONOXYLAN = X

GLUCOMANNAN♦ACE TYL= GM♦ Ac

X=8

GA=2 X=6

GA=1 X=6

I""

E=1 L=1 X=3

11*1

E=1 L=1 X=3 GM=4

C=39

C=39

GM*Ac=

30

CELLULOSE=C

Fig. 7-17.

C=39

Material balance for a typical two-stage cook of Scots pine (Sjöström et a/., 1962).

pulp is treated in the digester at a higher pH with sodium carbonate solution in order to remove hemicelluloses. In the case of pine wood, which cannot be delignified directly with acid sulfite, a neutral or alkaline pretreatment is needed as the first cooking stage. 7.2.7

Reactions of Extractives

During sulfite pulping the fatty acid esters are saponified to an extent determined by the conditions. Some of the resin components can also be­ come sulfonated, resulting in increased hydrophilicity and better solubility. However, the partial removal of resin that always occurs during sulfite cook­ ing and subsequent mechanical treatment is mainly associated with the formation of finely dispersed resin particles in stable emulsions. The dis­ solved lignosulfonic acids act as detergents with respect to the lipophilic resin components. Acid sulfite pulping causes terpenes, terpenoids, and flavonoids to be­ come partially dehydrogenated. The formation of p-cymene from a-pinene

136

Chapter 7. Wood Pulping

è —$ OH

OH

Fig. 7-18. Conversion of a-pinene to p-cymene and taxifolin to quercetin during acid sulfite pulping.

and quercetin from taxifolin are well-known reactions of this type (Fig. 7-18). Due to unsaturation, diterpenoids, including the resin acids, are probably partially polymerized to high molecular weight products causing pitch problems in subsequent pulp handling. Delignification of parenchyma cells with a high content of resin remains incomplete. 7.2.8

Side Reactions

A considerable part of the hydrogen sulfite ions is consumed in reactions other than the sulfonation of lignin. In the absence of wood, sulfur dioxide solutions are decomposed at elevated temperatures according to the equa­ tion: 4 HSCV - * S 2 0 3 2 - + 2 S0 4 2 ' + 2 H + + H O

(7-9)

This disproportionation reaction follows complex kinetics involving forma­ tion of polythionates ( S 3 0 ^ ~ , S 4 0 ^ ~ , and S 5 0^~) as intermediate prod­ ucts. The initial and rather slow decomposition during the induction period is later accelerated by thiosulfate, which functions as an autocatalyst. After a critical thiosulfate concentration has been reached, sulfur is precipitated, and the acidity increases rapidly. Another mechanism giving rise to thiosulfate formation is the reduction of hydrogen sulfite by wood components. Sugars play an important role in this reduction (see p. 133). Similarly, a-pinene is oxidized to p-cymene, formic acid to carbon dioxide, and taxifolin present in Douglas fir heartwood to quercetin. Sulfite cooking liquors, however, contain less thiosulfate than might be expected on the basis of these side reactions (Table 7-5). This discrepancy can be attributed to continuous consumption of thiosulfate in reactions with lignin, producing thioether cross-links (see p. 129). This type of sulfur, termed "organic excess sulfur," may account for 5 - 1 0 % of the total

7.2 Sulfite Pulping TABLE 7-5.

Origins of Thiosulfate Formation during Sulfite Pulping-3

s 2 cv-

(g/liter)

Cause Aldonic and sugar sulfonic acid formation (5% sugar composition, wood basis) Carbon dioxide formation (0.3% formic acid, wood basis) Cymene formation (0.5 kg ptp) Sulfate formation ( 2 - 3 g/liter) Total thiosulfate formation Found as thiosulfate Found as polythionate (expressed as thiosulfate) Total thiosulfate found a

137

Sulfur loss (kg/ton of pulp)

4.0

23

0.9

5

0.02 1.5 6.5 0.5-2 0.5-1.5 1-3.5

— 9 37

5-20

From Rydholm (1965).

organically bound sulfur. The major portion, 8 0 - 9 0 % of the sulfur, exists in the form of sulfonate groups in the lignin, although minor amounts of sulfite are also consumed in the formation of carbohydrate sulfonic acids. 7.2.9

Composition of Sulfite Spent Liquors

In addition to lignosulfonates and hemicelluloses and their degradation products, sulfite spent liquors contain small amounts of uronic acids, methyl TABLE 7-6. Typical Composition of the Sulfite Spent Liquor Resulting from the Acid Sulfite Pulping of Norway Spruce

Component Lignosulfonates Carbohydrates Arabinose Xylose Mannose Galactose Glucose Aldonic acids Acetic acid Extractives Other compounds

Content (% of dry solids)

Composition (% of carbohydrates)

55 28 4 22 43 17 14 5 4 4 4

138

Chapter 7. Wood Pulping

glyoxal, formaldehyde, methyl alcohol, furfural, etc. Most of the remaining sulfur dioxide in the liquor is directly titratable, but some of it is liberated slowly under titration or after certain treatments. The a-hydroxysulfonic acids derived from carbonyl compounds are responsible for this "loosely combined sulfur dioxide'' (cf. Figs. 2-28 and 7-8). In the liquors from acid sulfite cooking most of the carbohydrates are present in the form of monosaccharides (Table 7-6, cf. also Table 10-1). After bisulfite and neutral sulfite pulping, however, a large portion of the sugars remains as oligo- and polysaccharides. Characteristic of the spent liquors from neutral sulfite cooking of hardwood is the high proportion of acetic acid in comparison with the other organic constituents present. 7.2.10

Recovery and Conversion of Sulfite Cooking Chemicals

In addition to the organic solids resulting from the degradation and dis­ solution of the wood constituents the spent cooking liquors contain the inorganic pulping chemicals, which for the most part have been changed during the pulping reactions. A variety of useful products can be produced from this organic source (see Section 10.2), but most of the solids in the spent liquor are still burned with generation of heat. Beyond the heat econo­ my aspects, the combustion of the organic substance is today necessary from the pollution point of view. Because calcium was long used almost exclusively in the sulfite process as the base, no need existed for the recovery of this comparatively cheap chemical. Interest was therefore only directed toward the relatively simple recovery of excess sulfur dioxide. However, the introduction of the more expensive soluble bases, together with more stringent environmental re­ quirements, stimulated the development of methods for the recovery of both heat and inorganic chemicals (base and sulfur). The concentration of solids in the sulfite spent liquors in the digester after cooking is 1 1 - 1 7 % , dropping to 1 0 - 1 5 % after pulp washing. For proper ignition and burning, the spent liquors are in most cases concentrated in multiple-effect evaporators to a solids content of 5 0 - 6 5 % . Because volatile components, such as acetic and formic acids and furfural, are transferred to the steam condensates upon evaporation, the handling of these dilute li­ quors requires specific procedures to avoid water pollution. After combus­ tion the resulting ash, consisting of a mixture of calcium sulfate and calcium oxide in roughly equal amounts, is not converted for reuse in pulping; only the dust is collected to reduce air pollution. The stack gases resulting from the combustion of calcium-based spent liquors are especially problematic,

7.2 Sulfite Pulping

139

because of their high content of sulfur dioxide which can be neither eco­ nomically recovered nor eliminated. A furnace similar to the Tomlinson kraft recovery furnace is used for the combustion of magnesium-based sulfite spent liquors. In this case, however, no smelt is obtained; instead the base is completely recovered as magne­ sium oxide in dust collectors; the sulfur escapes as sulfur dioxide and is absorbed from the combustion gases in scrubber towers. However, because magnesium hydroxide has a very low solubility in water, a complete recov­ ery of sulfur dioxide meets difficulties. Ammonium-based sulfite spent liquors can be burned in the same type of furnace as the calcium-based liquors. However, during combustion the base is decomposed to form nitrogen and water and the problems with fly ash are thus eliminated. All sulfur escapes to the combustion gases as sulfur dioxide which can be partly absorbed in an ammonia solution. Sodium-based spent liquors both from the acid and neutral sulfite pro­ cesses can be burned in a kraft type furnace. A smelt is obtained, consisting of sodium sulfide and sodium carbonate. The sulfur-to-sodium molar ratio is about 1:1 for sulfite spent liquors instead of around 0.15:1 in kraft black liquors, which means that a considerable part of the sulfur escapes as sulfur dioxide. Sodium carbonate from the recovery cycle is suitable for the ab­ sorption of the sulfur dioxide, although special problems are encountered because the sulfur dioxide concentration of these gases is low and oxidation to sulfur trioxide must be avoided. However, the greatest problem is the complete removal of sulfide from the smelt and its conversion to pure cookW00D

I

PULP

Spent liquor

EVAPORATION

S02

COMBUSTION

S02(aq » NaHSO3

ABSORPTION •**>2

I

Na,C0 BURNING

VSH2S

Na2S

NaHC03

I co2 Fig. 7-19.

Recovery and conversion cycles for sulfite cooking chemicals.

140

Chapter 7. Wood Pulping

ing chemicals. Especially for acid sulfite pulping, the cooking chemicals must be very pure, since other sulfur compounds, especially thiosulfate, are detrimental for pulping. Several recovery systems based on the use of a conventional kraft recovery furnace have been developed. In another and a simpler system, which, however, requires a modified furnace construction, all sulfur is first converted to hydrogen sulfide together with partial or com­ plete gasification of the organic constituents. Sodium is thus recovered as pure carbonate. Hydrogen sulfide is finally converted to sulfur dioxide and absorbed. The recovery and conversion cycles for sulfite cooking chemicals are illustrated in Fig. 7-19.

7.3

Kraft Pulping

7.3.1

Cooking Chemicals and Equilibria

Kraft pulping is performed with a solution composed of sodium hydroxide and sodium sulfide, named "white liquor/' According to the terminology the following definitions are used, where all the chemicals are calculated as sodium equivalents and expressed as weight of NaOH or N a 2 0 . Total alkali Titratable alkali Active alkali Effective alkali

All sodium salts NaOH + Na2S + Na2COa NaOH + Na2S NaOH + \ Na2S

Causticizing efficiency

—FrT~ % NaOH + Na2C03 Na2S 10 % ° NaOHa:+SNa Na22SS Na2S 100 ——£-——— % Na2S + Na2S04

Sulfidit

*

Degree of reduction

100

In modern pulping chemistry weight units of NaOH are often replaced by molar units, e.g., moles of effective alkali per liter of solution or kilogram of wood. Table 7-7 shows typical conditions for kraft pulping. The charge of effective alkali (NaOH) applied is usually 4 - 5 moles or 1 6 - 2 0 % of wood. The following equilibria are involved in the aqueous solutions containing sodium sulfide and sodium hydroxide: S2- + Η 2 θ ^ HS- + HO-

(7-10)

HS" + H 2 0«± H2S + HO"

(7-11)

7.3 Kraft Pulping TABLE 7-7.

Alkaline Pulping Methods and Conditions

Method Alkali (soda) Kraft Soda-anthraquinone Prehydrolysis-kraft Prehydrolysis stage Kraft stage a b

141

Max. temp. (°C)

Time at max. temp.

pulp yield

"Base"

Active reagents

(hr)

(%)

13-14 13-14 13-14

Na + Na + Na +

HOHS-, H O ­ HO-, AHQb

155-175 155-175 160-175

2-5 1-3 1-3

50-703 45-55 45-55 35-40

3-4 13-14

Na +

H+ HS-, H O -

160-175 155-175

0.5-3 1-3

pH range

Softwood

Hardwood. Anthrahydroquinone.

The equilibrium constants for these reactions are: K, = [HS"] [ H O - ] / [ S 2 - ]

(7-12)

K, = [H,S] [ H O " ] / [HS-J

(7-13)

Since ΚΛ ~ 10 and K2 ~ 1 0 ~ 7 , the equilibrium in equation (7-10) strongly favors the presence of hydrogen sulfide ions and for all practical purposes, sulfide ions can be considered to be absent (Fig. 7-20). The concentration of hydrogen sulfide becomes significant below pH 8 and needs to be consid­ ered only in modified kraft pulping processes involving pretreatment at low pH (see Section 7.3.6).

1

o 10

2

~v

\H*

/

\^

z io4

o

<

HO/

/

/\/ί'

o

o

' J

ir-

H 25

io· 1

£

^"

6

'

8 PH

*

1

1

10

12

X,

H

1

1

END

START

Fig. 7-20. Equilibrium diagram showing the composition of the kraft white liquor at different pH values. Concentration, 0.2 mole/liter; temp., 25°C. Based on equilibrium constants corre­ sponding to Κλ = 10 and K2 = 1 0 - 7 .

142

7.3.2

Chapter 7. Wood Pulping

Impregnation

In the kraft process thorough impregnation of the chips with cooking chemicals is not as critical as in acid sulfite pulping. The diffusion of chem­ icals in liquid-saturated wood is controlled by the total cross-sectional area of all the capillaries. In moderately alkaline solutions (pH < 12.5) the effec­ tive capillary cross sectional area (ECCSA), which is the area of paths avail­ able for diffusion, is higher in the longitudinal direction than in the radial and tangential directions (Fig. 7-21). However, because of swelling caused by alkali at pH values above 13, the ECCSA is increased in tangential and radial directions, approaching the same permeability as in the longitudinal direction.

, to to

0.6

g

0.5

|


Longitudinal

_J

d <

Û.UJ

03

< cr " u <

^ o i UJ »- 0.1 ÜL U

U_ UJ LU tO

0

9

10

11

PH

12

13

H

Fig. 7-21. Effective capillary cross-sectional area (ECCSA) of aspen wood as a function of pH (Stone, 1957). © 1957. TAPPI. Reprinted from Tappi 40(7), p. 54, with permission.

7.3.3

General Aspects of Delignification

The consumption of effective alkali in a kraft cook corresponds to about 1 50 kilogram sodium hydroxide per ton of wood. As a result of the alkaline degradation of polysaccharides, about 1.6 equivalents of acids are formed for every monosaccharide unit peeled from the chain. Of the charged alkali, 6 0 - 7 0 % is required for the neutralization of these hydroxy acids, while the rest is consumed to neutralize uronic and acetic acids (about 10% of alkali) and degradation products of lignin (25-30% of alkali). Hydrogen sulfide ions react with lignin, but most of the sulfur-containing lignin products are decomposed during the later stages of the cook with formation of elemental sulfur, which combines with hydrogen sulfide ions to

7.3 Kraft Pulping

143

form polysulfide. However, kraft lignin still contains 2 - 3 % of sulfur, corre­ sponding to 2 0 - 3 0 % of the charge. As already shown (cf. Fig. 7-4), the selectivity of delignification is rather low for kraft pulping. The carbohydrates are attacked already at a com­ paratively low temperature (Fig. 7-22). This means that the acetyl groups are completely removed and the primary peeling process is terminated long before the maximum cooking temperature has been attained. The reactivity of the polysaccharides varies depending on their accessibility as well as on their structure. Because of its crystalline nature and high degree of poly­ merization, cellulose suffers less losses than the hemicelluloses. The dissolution of lignin can be divided into three phases (Fig. 7-23). The initial phase of delignification takes place at temperatures below 140°C and is controlled by diffusion. Above 140°C, the rate of delignification becomes controlled by chemical reactions and accelerates steadily with increasing temperature. The rate of lignin dissolution remains high during this "bulk delignification" phase, until about 90% of the lignin has been removed. The final slow phase is termed "residual delignification" and can be regulated to some degree by varying the alkali charge and the cooking temperature. The kinetics of the delignification are of importance especially when considering the control of the pulping process. Since kraft pulping follows simpler kinetics than the sulfite processes, more applications have been adopted for this case. Because of the heterogenity of the system, however,

1

2

3

4

5

COOKING TIME (hr) Fig. 7-22. Dissolution of carbohydrates (1) and lignin (2) during sulfate (—) and soda (—) pulping of Norway spruce (see Enkvist et a/., 1957). Note that the cooking temperature is exceptionally low (140°C), resulting in much less dissolution than in normal pulping.

144

Chapter 7. Wood Pulping

O O

„Initial delignification /Bulk delignification

o

< ID g

Residual /delignification

10 LU OC

10 100 500

1000

1500

2000

H FACTOR Fig. 7-23. Removal of lignin during kraft pulping of pine (—) and birch (—) as a function of the H factor (see Kleppe, 1970). © 1970. TAPPI. Adopted from Tappi 53(1 ), p. 37, with permission.

pulping reactions are complicated and can therefore not be treated in the same fashion as homogeneous reactions in solution. The overall rate of the bulk delignification in kraft pulping, during which the variations in hydroxyl and hydrogen sulfide ion concentrations are mod­ erate, follows pseudo-first-order kinetics, approximately in conformity with the following equation: (7-14)

dt

where L is the lignin content of wood residue at time t and k the rate constant. Based on experimental data of k at varying temperatures, the value of the activation energy £a can be calculated from the Arrhenius equation: In k = In A -

-—

(7-15)

where 7 is the absolute temperature (Kelvin), R the gas constant, and A a further constant including the frequency factor. The activation energy Ea for kraft delignification of softwood has been determined to be 130-150 kj/mole (31-36 kcal/mole) (bulk phase) and about 50 and 120 kj/mole (12 and 27 kcal/mole) for the initial and final phases, respectively. According to a simplified system the net effect of both cooking time and

7.3 Kraft Pulping

145

temperature can be expressed by means of a single variable. In this system the rate at 100°C is chosen as unity and rates at all other temperatures are related to this standard. When using a value of 134 kj/mole (32 kcal/mole) for f a the rates at any other temperatures can then be expressed by the following equation: In (relative reaction rate) = 43.2 -

—'■=—

(7-16)

The time integral of the relative reaction rate is called the H factor: H = Γ

exp(43.2 - ^y-)dt

(7-17)

A normal heating period contributes to the H factor by 150-200, and 1 5 0 0 2000 are needed for a complete kraft cook. Within the bulk delignification phase the relative reaction rate is doubled when the temperature is increased by about 8°C. If the hydroxide and hydrogen sulfide ion concentrations vary in a reproducible manner the H factor will predict the degree of delignifica­ tion with sufficient precision. 7.3.4

Lignin Reactions

As in sulfite pulping, depolymerization of lignin depends on the cleavage of ether linkages, whereas the carbon-to-carbon linkages are essentially stable. The presence of hydrogen sulfide ions greatly facilitates delignifica­ tion because of their strong nucleophilicity in comparison with hydroxyl ions. Cleavage of ether linkages, promoted by both hydroxyl and hydrogen sulfide ions, results also in increasing hydrophilicity of lignin because of the liberation of phenolic hydroxyl groups. The degraded lignin is dissolved in the cooking liquor as sodium phenolates. Studies with model substances representing various structural units in lignin have largely clarified the delig­ nification reactions in kraft pulping. Etherified Phenolic Structures Containing ß-Aryl Ether Bonds In etherified p-phenolic structures the ß-aryl ether linkage is cleaved by hydroxide ions according to the mechanism shown in Fig. 7-24. The reaction proceeds via an oxirane intermediate which is subsequently opened with formation of an α,β-glycol structure. This reaction promotes efficient delignification by fragmenting the lignin and by generating new free phenolic hydroxyl groups. Free Phenolic Structures Containing ß-Aryl Ether Bonds The fi rst step of the reaction involves the formation of a quinone methide from the phenolate anion by the elimination of a hydroxide, alkoxide, or phenoxide ion from

146

Chapter 7. Wood Pulping

1

Θ


^OCH-, ^0

Fig. 7-24. Cleavage of ß-aryl ether bonds in nonphenolic phenylpropane units during soda pulping (Gierer, 1970). H3CC\

H3CCX

\-o-0- H-OHC

'

HC

0CH

3

o

'

T^œH3

2 H C0 3 V_ [Ç*Ô~(/ V


<

„CH

eg

I

I

%-CH

CH

3b

CH II

£

5

II

„ o 170 C

ÇH

—- P

6

Fig. 7-25. Main reactions of the phenolic ß-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or for­ maldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrogen sulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the ß-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur. I

I

CH

CH

J V - ^«~ « w CH 2 i 5l <,Θ


Hjj-R

R

J M H3co^r^ io

CH

-

CH

>°

H3C0

ιόι'

Fig. 7-26. Elimination,of proton and formaldehyde from the quinone methide intermediate during alkali pulping (Gierer, 1970).

7.3 Kraft Pulping

147

the α-carbon (Fig. 7-25). The subsequent course of reactions depends on whether hydrogen sulfide ions are present or not. In the latter case (soda pulping), the dominant reaction is the elimination of the hydroxymethyl group from the quinone methide with formation of formaldehyde and a styryl aryl ether structure without cleavage of the ß-ether bond (Fig. 7-26). When hydrogen sulfide ions are present (strong nucleophiles) they react with the quinone methide to form a thiol derivative which is converted to a thiirane structure with simultaneous cleavage of the ß-ether bond. The thiirane can be dimerized to a dithiane structure, but this as well as other sulfur-containing intermediates are decomposed, forming elemental sulfur and unsaturated side-chain structures. Competing reaction paths are pos­ sible, giving rise to other minor degradation products, such as guaiacol. Structures Containing a-Ether Bonds The a-ether bonds in phenolic phenylcoumaran (Fig. 7-27) and pinoresinol structures are readily cleaved by hydroxide ions, usually followed by the release of formaldehyde. Only in the case of open a-aryl ether structures does this reaction result in the fragmentation of lignin. In contrast, the a-ether bonds are stable in all etherified structures. Methoxyl Groups Lignin is partially demethylated by the action of hy­ drogen sulfide ions forming methyl mercaptan which is convertible to di-

' ?H 0 —ÇH

β

_ _

H3CO^Ai-H 0Θ i n

H

--.

3 C 0 ^ C CH CH

0CH3 0

0*

2

3

Fig. 7-27. Example of the base-catalyzed reactions of the free phenolic phenylcoumaran structures (1) (Gierer, 1970). Cleavage of the a-aryl ether bond results in a quinone methide intermediate (2) which after elimination of a proton from the ß-position is stabilized to a stilbene structure (3). Structures containing open a-aryl ether bonds react analogously.

Ό10Θ1

/010Θ)

L 02(R = H) CH3SSCH3

Fig. 7-28. Cleavage of methyl aryl ether bonds with simultaneous formation of methyl mer­ captan (CH 3 SH), dimethyl sulfide (CH3SCH3), and dimethyl disulfide (CH 3 SSCH 3 ) during kraft pulping. R = H or methyl group.

148

Chapter 7. Wood Pulping

methyl sulfide by reaction with another methoxyl group. In the presence of oxygen, methyl mercaptan can be oxidized further to dimethyl disulfide (Fig. 7-28). Because the hydroxide ions are less strong nucleophiles than hydrogen sulfide ions, only small amounts of methanol are formed. Methyl mercaptan and dimethyl sulfide are highly volatile and extremely mal­ odorous, causing an air pollution problem that is difficult to master. Condensation Reactions A variety of condensation reactions are known to occur in alkaline pulping. Since carbon-to-carbon linkages are formed between lignin entities, it has been proposed that as a result of condensation reactions, lignin dissolution is retarded, particularly during the terminal phases of kraft pulping. It has been suggested that the major part of condensation processes occurs at the unoccupied C-5 position of phenolic units. Thus, in isolated MWL preparations about half of the C-5 positions are unsubstituted, while in isolated kraft lignins only about one third of these positions remain free. The syringyl units of hardwood lignins cannot, of course, undergo condensation reactions of this type. Figure 7-29 shows some examples of postulated condensation reactions forming diarylmethane structures of three different types. In the first case (A) a phenolate adds to a quinone methide structure, forming an a-5 linkage. The second case (B) illustrates a similar condensation between the Ί - and a-

H3co-

H3C0

0CH3

/0CH 3

H-O-CH KC--cP

' Ύ '

I

HrfxrV-

\=Γ

l_ CH20

Fig. 7-29.

^OCH3

;

H,co^y-

l_

71

; k 0 H3C0-V"

l_

^H3C0-V-CH2Jy-(

Examples of condensation reactions during alkali and kraft pulping (Gierer, 1970).

7.3 Kraft Pulping

149

carbons with simultaneous removal of the propane side chain. The third reaction (C) involves formaldehyde released from the 7-carbinol groups (see Fig. 7-26) and also leads ultimately to a diarylmethane structure. Formation of Chromophores During kraft delignification the color of the chips darkens gradually until a yield level of 6 0 - 7 0 % is reached (light absorption coefficient of ca. 40 m 2 /kg instead of ca. 5 - 1 0 m 2 /kg for the original wood at 457 nm). Further delignification brings about a modest brightening of the pulp. The specific light absorption coefficient of the re­ sidual lignin, however, increases continuously, reaching ca. 500 m 2 /kg at the end of the pulping. For comparison, the corresponding value for wood lignin is 2 0 - 4 0 m 2 /kg. The color of unbleached pulps is caused by certain unsaturated structures (chromophores). In addition, leucochromophores, which can be converted into chromophores by air oxidation may be present in the pulp. Most of the chromophores are presumed to be derived from lignin (Fig. 7-30) although some chromophoric groups can also be introduced into the polysaccharides, for example, carbonyl groups.

Fig. 7-30. Examples of proposed leucochromophoric and chromophoric structures. Arylcoumarones (1) and stilbene quinones (2) are thought to be formed from stilbenes after oxida­ tion. Butadiene quinones (3) could arise from oxidation of hydroxyarylbutadienes being formed from phenolic pinoresinol structures during kraft or neutral sulfite pulping. Cyclization may yield intermediates which are further oxidized to cyclic diones (4). A resonance-stabilized structure (5) results from the corresponding condensation product formed during pulping, oQuinoid structures (7) are oxidation products of catechols (6) formed during alkaline or neutral pulping processes.

150

7.3.5

Chapter 7. Wood Pulping

Reactions of the Polysaccharides

Because of the alkaline degradation of polysaccharides kraft pulping re­ sults in considerable carbohydrate losses. The acetyl groups are hydrolyzed at the very beginning of the kraft cook (from hardwood xylan and softwood galactoglucomannans). In the earlier stages of cooking the polysaccharide chains are peeled directly from the reducing end groups present (primary peeling). As a result of the alkaline hydrolysis of glycosidic bonds, occurring at high temperatures, new end groups are formed, giving rise to additional degradation (secondary peeling) (cf. Section 2.5.5). As a consequence, the yield of cellulose is always reduced in kraft pulping, although to a lesser extent than that of the hemicelluloses which are degraded more extensively due to their low degree of polymerization and amorphous state (cf. Table 7.4). The peeling reaction is finally interrupted because the competing "stopping reaction" converts the reducing end group to a stable carboxylic acid group. Mechanism of the Peeling Reaction The carbohydrate material lost in peeling is converted to various hydroxy acids. In addition, formic and acetic acid and small amounts of dicarboxylic acids, are formed as well. Figure 7-31 (lower portion) shows a simplified reaction scheme illustrating the mechanism of formation of the main degradation products. The rearrange­ ment of a reducing end group to a 2-keto intermediate is followed by ßalkoxy elimination. The cleaved monosaccharide unit is rearranged into a 2,3-diulose structure from which either glucoisosaccharinic acid (cellulose and glucomannans) or xyloisosaccharinic acid (xylan) is formed via a benzilic acid rearrangement. The diulose structure can also be cleaved by re­ versed aldol condensation to glyceraldehyde, which is then converted via methylglyoxal to lactic acid. Finally, a probable route for the formation of 3,4-dideoxypentonic and 2-hydroxybutanoic acids proceeds via formic acid elimination from the 3-keto intermediate, followed by a benzilic acid rear­ rangement. Figure 7-32 illustrates the glucomannan and xylan losses during kraft pulping of pine wood. As can be seen, an appreciable portion of the lost

Fig. 7-31. Peeling and stopping reactions of polysaccharides (Sjöström, 1977). R = polysac­ charide chain and R' = C H 2 O H (cellulose and glucomannans) or H (xylan). Cellulose and glucomannans (R' = C H 2 O H ) : 3-Deoxyhexonic acid end groups (metasaccharinic acid) (1), 2-C-methylglyceric acid end groups (2), 3-deoxy-2-C-hydroxymethylpentonic acid (glucoiso­ saccharinic acid) (3), 2-hydroxypropanoic acid (lactic acid) (4), and 3,4-dideoxypentonic acid (2,5-dihydroxypentanoic acid) (5). Xylan (R' = H): 3-Deoxy-2-C-hydroxymethyltetronic acid (xyloisosaccharinic acid) (3), 2-hydroxypropanoic acid (lactic acid) (4), and 2-hydroxybutanoic acid (5). © 1977. TAPPI. Adopted from Tappi 60(9), p. 152, with permission.

7.3 Kraft Pulping

CHO I CHOH HOCH I HC—OR I HCOH

CHO I COH

UH I

HC—OR HCOH I R'

i CH29 OH I CO HOCH I HC—OR HC OH

CH^OH CHOH

CHO I CO

I

ÇH 2

HC—OR HCOH I R' CH20 OH I CHOH

- CHO io I HC—OR I H 0 C—OR

co I

HeOH

COOH I CHOH I CH 2 HC—OR HCOH I R'

1

CMo I3 CO

COOH I ^OH ?^CH2 H Q C—OR

H 2 C—OR

ROH CHoOH I 2 CO ->->

I

HOC

II

i

CH«OH I 2 CO

CH2 HCOH

I

CO CH 2 OH CHO I HCOH CH 2 OH

HC HCOH I R'

co I

CH 2 OH

-

CHO I COH II CH 2

CHO I CO I CH 3

COOH I CHOH I CHo

COOH I/OH ^CH2OH

I

2

HCOH I R'

R'

! CHO CH 2 OH I CHOH CO I CO > CHZ2 I I CH2 - HCOOH HCOH I HCOH R' I R'

CH 0 OH I CO I CH II CH I R'

CHO

I

CO

I

H CH 22 CH

COOH

I

CHOH I CH 2 CH 2

151

152

Chapter 7. Wood Pulping

UJ O '* 10 O ^

*

,o

g1 - 12 - o

3£io UJ »■ li_

o Z

o

8

6

5

*

ί

2

GO

Q 10

20

30

40

50

60

TOTAL YIELD LOSS (·/. of wood) Fig. 7-32. Hemicellulose losses during sulfate pulping of pine wood (Sjöström, 1977). © 1977. TAPPI. Reprinted from Tappi 60(9), p. 152, with permission.

xylan is actually not degraded but dissolves in the cooking liquor as a polysaccharide. The amount of dissolved xylan reaches a maximum around the midpoint of the delignification process. Most of the carbohydrate losses take place during the heating-up period; hence, alkaline hydrolysis of glycosidic bonds does not appreciably contrib­ ute to the initial loss. As can be seen from the data in Table 7-4, more than 30% of the wood polysaccharides is lost during kraft pulping. The yield loss is especially high for glucomannan (which is present as galactoglucomannans in the original wood), but cellulose losses occur as well. The relatively high carbohydrate yield for hardwood kraft pulp compared with hardwood sulfite pulp and softwood kraft pulp depends on the fact that hardwood contains xylan as the dominant hemicellulose component, and xylan is comparatively resistant under alkaline pulping conditions. Mechanism of End Group Stabilization The main stopping reaction routes are presented in the upper portion of Fig. 7-31. The dominant route is initiated by a β-hydroxy elimination directly from the aldehydic end groups. The resulting dicarbonyl intermediate is converted to a metasaccharinic acid end group via a benzilic acid rearrangement. The end groups can also rearrange to a 3-keto intermediate, which then loses the 5- and 6-carbons as glycolaldehyde in a reversed aldol condensation. The rest of the end group undergoes ß-hydroxy elimination followed by a benzilic acid rearrange­ ment. As a result, a 2-C-methylglyceric acid end group is formed. In addi­ tion to the metasaccharinic and 2-C-methylglyceric acid end groups, small amounts of 2-C-methylribonic (glucosaccharinic) .and aldonic acid end

7.3 Kraft Pulping

m

COOH

COOH

COOH

J~°\

"î^o.

>-α

U)H ^ Η . Ο Η OR

—

\_/H.OH

^

HO-U

>H.OH

OR

153

COOH

J-o. ^

0=(

OR

>H.OH OR

I COOH

J-°\ 0=(

>H.OH ♦ ROH

Fig. 7-33. Alkaline degradation of 2-0-(a-L-rhamnopyranosyl)-D-galacturonic acid (Johans­ son and Samuelson, 1977). R represents the rhamnose group in the xylan chain.

groups have been found to be present. The presence of aldonic acid end groups indicate that some oxidative reactions also occur. In the case of pulping with alkali alone (soda process), the oxidation might depend on the presence of dissolved oxygen, whereas the polysulfides generated during kraft pulping can function as oxidants. Softwood xylan is partially substituted with arabinose at the C-3 position of the xylose units. During the course of peeling, arabinose is easily elimi­ nated from the chain under simultaneous formation of a metasaccharinic acid end group (ß-alkoxy elimination) which stabilizes the chain against further alkaline peeling. However, because of its relatively low content in softwood, xylan makes only a small contribution to the total carbohydrate yield in pulping. Much more important is the behavior of hardwood xylan during kraft pulping. In this connection, the detailed structure of the xylan chain is of interest (Fig. 3-17). The terminal xylose unit is rapidly cleaved from the xylan chain, but the remaining galacturonic acid end group is stable against further peeling. Its stability is not permanent, however (Fig. 7-33). After hydroxy elimination at C-3, a 2-enuronic acid group is formed which is decomposed at higher temperatures ( > 100°C) after isomerization of the double bond to the C-3-C-4 position. The remaining terminal rham­ nose unit is eliminated very easily because its C-3 position is bound to the following xylose unit. The 4-O-methylglucuronic acid groups prevent the peeling of xylan chains at lower temperatures ( < 100°C) but they offer only a partial protec­ tion at higher temperatures. Since the 4-O-methylglucuronic acid groups are bound to the C-2 position in the xylose units, no conversion of this carbon atom to a carbonyl group can take place. Instead, HO-3 is eliminated di­ rectly (ß-hydroxy elimination). Significance of Uronic Acid Groups The uronic acid content is much lower in the final kraft pulp than in the original wood. In the glucuronoxylan remaining in the pulp it corresponds to a molar ratio of roughly 1:25

154

Chapter 7. Wood Pulping

►Hrf> C H 3 OH

H3co

-Λ

0

OH

OH

OH

Fig. 7-34. Loss of 4-O-methylglucuronic acid groups (Johansson and Samuelson, 1977). P denotes fragmentation products formed.

(glucuronic acid/xylose), whereas this ratio for native xylan is about 1:5 (softwood) and 1:10 (hardwood). Since xylan is not evenly substituted by the uronic acid groups it is probable that the fractions of high uronic acid content are preferably dissolved, resulting in a lower uronic acid content of the pulp. Another reason for the decrease in the uronic acid content is the cleavage of these groups from the xylan chain, since the pyranosyluronic acid linkages are more sensitive to alkaline hydrolysis than are the corre­ sponding glycosyl linkages. An additional reaction possibly proceeds via CH3O elimination at C-4 (ß-position to the carboxyl group) followed by H-5 elimination (Fig. 7-34). Other Reactions In addition to the direct stabilizing effect of the uronic acid groups, the relatively high yield of hardwood kraft pulp is also due to the readsorption of xylan on the fibers (Fig. 7-35). After kraft pulping of softwood, the glucomannan remaining in the pulp still contains traces of 200

Temperature Q ÜJ CO

15

/

or o

CO Q

-M50 U

/

10

< z <

Birch

or

Z)

/

/

5

/

-

^

/

100 < or

LU O-

1

50

Spruce

Σ

LU V-

y^y^^. 1

2

3

A

5

6

7

COOKING TIME (hr) Fig. 7-35. Adsorption of xylan on cotton fibers present in the digester during kraft pulping (Yllner and Enström, 1956).

7.3 Kraft Pulping

155

galactose residues, and the xylan has some arabinose residues contrary to sulfite pulping during which these moieties are cleaved completely. 7.3.6

Stabilization of Polysaccharides against Alkaline Degradation

The primary peeling of polysaccharides by alkali can be avoided by the elimination of the aldehyde functions from the end groups. The reduction of these groups to alcohols by sodium borohydride inhibits primary peeling, and the carbohydrate yield is thus increased considerably. The end groups can also be stabilized by oxidizing them to carboxyl groups or by conver­ sion to other stable derivatives. Of the stabilization methods, the polysulfide pulping process is of prac­ tical importance. The influence of polysulfides is based on a specific oxida­ tion of the end groups to carboxyl groups via glucosone intermediates (cf. Section 8.1.3). Polysulfides can be prepared by catalytic oxidation of sulfide in the white liquor or by adding elemental sulfur into the kraft cooking liquor: n S2" +

n

~

1

O, + (n - 1 ) H 2 0 ^ S„2- + {In - 2)HCT S2- + n S - » S 2 ; ,

(7-18) (7-19)

In the latter method an excess of sulfide is created because of the added sulfur. This must be regenerated to elemental sulfur in order to avoid high sulfidities. Of the reducing methods, the pretreatment of wood chips with hydrogen sulfide (140°C, pH ~ 7) might be technically feasible. During such a treat­ ment the aldehyde end groups are reduced to thioalditols according to the following equation: H\HSR—CHO <

>

-H,S R_CH(SH)2 7 = ±

- H 2 0 , H2S R—CH(OH)SH < H2S R—CHS —> —S

>

R—CH2SH

(7-20)

The increase in pulp yield may reach 8% on dry wood basis, but requires high pressures ( > 1000 kPa) and a large excess of hydrogen sulfide (ca. 10% of wood). Only a fraction of the hydrogen sulfide ( 1 - 2 % of wood) is con­ sumed and the rest is recoverable. Table 7-8 illustrates the influence of some oxidizing and reducing agents on the carbohydrate yield of kraft pulp. Sta­ bilization with anthraquinone is dealt with in Section 7.3.7.

156

Chapter 7. Wood Pulping

TABLE 7-8. Reductive and Oxidative Stabilization of Softwood Carbohydrates during Kraft Pulping Yield of polysaccharide (% of wood) Method/addition

Cellulose

Glucomannan

Glucuronoxylan

Total

Normal sulfate pulping Oxidation/polysulfide (4% S) Reduction/NaBH 4 Reduction/H 2 S

35 36 36 36

4 9 12 9

5 5 4 4

44 50 52 49

7.3.7

Sulfur-Free Pulping

In order to be able to reduce the pollution load of the pulp mills, attempts have been made to diminish the use of sulfur chemicals even if their com­ plete elimination is difficult. Sulfur contaminants are easily introduced into the system, e.g., in connection with the use of oil as fuel, and already traces of them give rise to odor. Alkaline solutions containing oxygen can be used for the removal of lignin from softwoods, but the delignification in this system is quite unselective. Better results have been obtained for hardwoods using pressurized oxygen at low alkalinities, e.g., sodium carbonate and sodium hydrogen carbonate solutions. Interest has also been directed toward a two-stage oxygen pulping system ("soda-oxygen pulping"). Here, wood chips are first subjected to soda pulping and then fiberized mechanically. The final delignification is carried out in the presence of alkali and oxygen. Sulfide in the kraft cooking liquor can be replaced, at least partly, by anthraquinone (AQ) or similar compounds which possess a marked ca-

CHO CHOH I

HO 9 ^ ""

C-0 COH I

Degraded lignin

^

^

ÇH0 CO I

Puinone methides

HO®^

—

Ç°2 ÇH0H I

^

Lignin

Fig. 7-36. Anthraquinone-anthrahydroquinone reactions with carbohydrates and lignin.

7.3 Kraft Pulping

157

Fig. 7-37. Cleavage of ß-aryl ether bonds in alkaline media by anthrahydroquinone with regeneration of anthraquinone.

pability of accelerating the delignification while at the same time stabilizing the polysaccharides toward alkaline degradation according to the mecha­ nism illustrated in Fig. 7-36. At moderate temperatures AQ is reduced to anthrahydroquinone (AHQ) by the polysaccharide end groups, which, in turn, are oxidized to alkali-stable aldonic acid groups. The reduced species or A H Q now acts as an effective cleaving agent with regard to the lignin ßaryl ether linkages in free phenolic phenylpropane units and is simul­ taneously oxidized to AQ (Fig. 7-37). The partly depolymerized lignin is further degraded by sodium hydroxide at elevated temperature. As a result of this reduction-oxidation cycle, additions as low as 0.01 % AQ of the dry wood weight markedly improve the delignification. Depending on the wood species, conditions, desired effect, etc., up to 0.5% may be used. 7.3.8

Reactions of Extractives

During kraft pulping the fatty acid esters are hydrolyzed almost com­ pletely although the waxes are much more stable than the fats. The fatty acids are dissolved together with the resin acids as sodium salts in the cooking liquor. Especially the resin acid soaps are effective solubilizing agents facilitating the removal of sparingly soluble neutral substances such as sitosterol in pine wood and betulinol and betulaprenols in birch wood. Because hardwoods do not contain resin acids, tall soap is usually added to the cook to reduce the content of extractives in the final pulp to a sufficiently low level so that the "pitch problems'' can be avoided. Some of the unsaturated fatty acids and resin acids are partly isomerized at kraft pulping conditions (for structures, see Table 5-2 and Fig. 5-8). Linoleic and pinolenic acids, representing dienoic and trienoic fatty acid types, are converted to the respective isomers with conjugated double bonds at 9,11 and 10,12 positions which are mainly of eis, trans configura­ tion. In the case of common resin acids, the principal change is a partial isomerization of levopimaric to abietic acid. The other members of the common resin acids are essentially stable at the kraft pulping conditions.

158

7.3.9

Chapter 7. Wood Pulping

Composition of Black Liquor

The organic material in the resulting black liquors after kraft pulping principally consists of lignin and carbohydrate degradation products in addi­ tion to a small fraction of extractives and their reaction products (Table 7-9). The black liquors are extremely complex mixtures containing a huge number of components of diversified constitutions and structures. The major portion of the lignin fraction constitutes of high-molecularweight material, which is precipitated when the liquor is acidified (cf. Sec­ tion 10.2.2). The composition of the "kraft lignins" is complex and it varies depending on the wood species and cooking conditions. However, some characteristic features are known of which examples are given in Table 7-10. Compared with native lignin, kraft lignin typically contains many more phenolic hydroxyl groups and carboxyl groups. Instead of the coniferyl type double bonds, which have been destroyed completely during cooking, new double bonds are present in the kraft lignin in styrene and stilbene strucTABLE 7-9. Typical Distribution of the Organic Material in Softwood (Pine) Kraft Black Liquors

Fraction/component Lignin d Hydroxy acids 0 Glycolic Lactic 3,4-Dideoxypentonic Glucoisosaccharinic ( 2-Hydroxybutanoic 3-Deoxypentonic Xyloisosaccharinic Others Formic acid Acetic acid Extractives Other compounds a

Content (% of dry solids)

Composition of hydroxy acid fraction (% of total acids)

46 30 10 15 10 35 5 5 5 15 8 5 7 4

About 90% of this fraction constitutes of polymeric material; minor amounts are monomeric and dimeric fragments (cf. Table 7-10 and Fig. 7-38). b For structures, see Fig. 10-8. c The ratio of threo (ß) and erythro (a) forms is about 2.5.

7.3 Kraft Pulping

159

TABLE 7-10. Characteristics of Softwood Kraft Lignin Compared with Milled Wood Lignin (MWL)

Characteristic

Kraft lignin

MWL

Molecular weight (Mn) Polydispersity (M w /M n ) Sulfur content (%) Functional groups3 Hydroxyl groups, total Guaiacyl OH Catechol OH Aliphatic OH Carboxyl groups Carbonyl groups

3000-5000 3-4 1-3

8000-10000 2-3

120 60 10 50 13 3

120 30

a

—

—

90

—

10-15

Per 1 0 0 C 6 C 3 units.

tures, which are formed during pulping. Many of the typical linkages origi­ nally present in native lignin have been extensively modified. Although the low-molecular-weight degradation products represent only a minor portion of the total lignin fraction, several hundreds of single compo­ nents have been identified. Of the monomeric compounds in softwood black liquors some are shown in Fig. 7-38. To the identified compounds CHO

OCK,

OCHfl

OCKL

CH2OH

OCH0

OCH0

Fig. 7-38. Abundant monomeric degradation product in softwood kraft black liquors. Guaiacol (1), vanillin (2), vanillic acid (3), acetovanillone (4), and dihydroconiferyl alcohol (5).

160

Chapter 7. Wood Pulping

belong also a number of hydroxylated monomeric arylalkanoic acids and dimeric hydroxy acids having the stilbene structure. Noteworthy is also that hydroxyalkanoic acids containing a guaiacyl or syringyl nucleus are present in softwood and hardwood black liquors. They are obviously condensation products between lignin fragments and degradation intermediates of carbo­ hydrates. The carbohydrate degradation products in black liquors consist of al­ iphatic carboxylic acids of which the hydroxy momocarboxylic acids are the dominant components. 7.3.10

Recovery and Conversion of Kraft Cooking Chemicals

In the kraft process large amounts of comparatively expensive cooking chemicals are used which has necessitated the development of an advanced technology for the recovery of these chemicals in combination with the generation of process energy. The total content of solids in pine kraft black liquor leaving the diffusers of filter washers is 1 5 - 2 0 % . It contains most of the degraded and dissolved wood material together with the inorganic chemicals. Most of the base has been consumed for the neutralization of the organic acids; sulfur is still predominantly present as hydrogen sulfide ions. After partial evaporation, the tall oil skimmings are recovered and treated separately (see Section 10.2.2). The content of solids in the concentrated black liquor coming from the multiple-effect evaporators and entering the Tomlinson-type recovery furnace is 5 0 - 7 0 % . The inorganic smelt remaining at the bottom of the furnace after combustion contains mainly sodium car­ bonate and sodium sulfide. A part of the substance, collected as dust and fumes from the cyclones and electrostatic precipitators at the top of the furnace, consists of sodium sulfate, and is returned to the concentrated black liquor for combustion. The stack gases further contain sodium sulfate in addition to organic sulfur compounds and sulfur dioxide. The losses of chemicals are compensated by adding sodium sulfate to the concentrated black liquor prior to its combustion. To avoid sulfur losses from the stack gases and also to reduce air pollution, the black liquor can be oxidized with air or gaseous oxygen before evaporation and/or combustion. The oxidation converts remaining sulfides to sulfates and mercaptans to disulfides and their further oxidation products. The smelt leaving the combustion furnace is dissolved in water and the sodium carbonate in the resulting "green liquor" is converted to hydroxide by lime: CaO + H 2 0 -+ Ca(OH).,

(7-21)

Ca(OH), + Na,CO, +± 2 NaOH + CaCO<3

(7-22)

7.4 Special Features of High-Yield Pumping

161

f WASHING

Black liquor

EVAPORATION

' N a 2S . N a O H

CAUSTICIZING

C a C 0 3/ Ca(OH)2

Fig. 7-39.

N a 2C 0 3 Na 2S

N a 2S 0 4 . N a 2C 0 3

Recovery and conversion of the kraft cooking chemicals.

In this procedure, calcium carbonate (lime sludge) is precipitated and sepa­ rated from the liquid. The remaining solution, consisting mainly of sodium sulfide and sodium hydroxide ("white liquor"), is used as such for cooking. After washing and drying the lime sludge is reburned to give new calcium oxide. Figure 7-39 illustrates the recovery and conversion of the kraft cooking chemicals. In the case of sulfur-free pulping (soda process, soda-oxygen process, and anthraquinone-alkali pulping) only sodium carbonate is re­ covered and chemical losses are compensated by adding sodium carbonate. The hydroxide-carbonate and lime cycles are the same as for the kraft pro­ cess.

7.4

Special Features of High-Yield Pulping

Although the lignin and carbohydrate reactions discussed in the previous sections are applicable to high-yield pulping, the chemistry associated with this type of processes deserves special attention. Material Losses In high-yield pulping processes, including CTMP, a treatment with alkaline sulfite solution is the most usual prestage prior to the final mechanical defibration. The material losses are low, but the handling of dilute effluents to avoid pollution is a special problem. Most of the resulting organic material consists of lignin and carbohydrate degradation products in addition to components from extractives. The average molecular weight of the dissolved lignin fraction has been found to be very low. Recent studies have revealed that fragments from the end group structures of lignin are released when softwood is treated at

162

Chapter 7. Wood Pulping

relatively mild conditions with neutral or alkaline sulfite solutions. The ma­ jor monomeric fragments consist of coniferyl alcohol and coniferyl al­ dehyde, of which only the latter is sulfonated. These observations suggest that sulfonation of coniferyl aldehyde structures is very fast in comparison with other lignin structures. Of the carbohydrates mainly hemicelluloses are degraded to soluble frag­ ments present in the effluents from high-yield pulping. Some of this material has not been degraded completely and it still exists as polysaccharides, but, in addition, various monomeric degradation products are present. This ma­ terial is mainly composed of aliphatic carboxylic acids, among which hydroxy carboxylic acids and acetic acid dominate. The hydroxy carboxylic acids are typical alkaline degradation products of polysaccharides. The ex­ tent of this type of degradation depends on the alkalinity and is still rather insignificant during high-yield pulping in comparison to kraft pulping (see Section 7.3.5). Most of the acetic acid originates from hemicelluloses, that is, either from softwood galactoglucomannans or from hardwood glucuronoxylans. Since the acetyl groups are extensively hydrolyzed already after a relatively short time of treatment, acetic acid is a prominent component particularly in effluents from high-yield pulping of hardwood. A direct mechanical processing of wood and defibration after its chemical pretreatment also results in some yield losses. Most of this material consists of finely divided fiber fragments ("fines") and colloidal particles, but some of the substance is dissolved. On prolonged washing more lignosulfonates are also leached out of the fibers. Of the hemicelluloses xylans are preferentially dissolved, and fragments of a surprisingly high degree of polymerization are present in effluents resulting from mechanical treatment of pulps. Acidic Groups The wood and the fibers liberated from it either by me­ chanical means or by chemical treatments carry a negative charge because of the presence of certain functional groups. These groups exhibit an acidic character provided that they can be ionized at ambient conditions. The positively charged counter ions (X + ), maintaining the balance of electroneutrality, are exchangeable to other cations (Y + ) present in the sur­ rounding solution: Fiber-X+ + Y+ *± Fiber-Y+ + Χ+

(7-23)

This ion exchange ability of wood and pulps leads to important conse­ quences, which must be taken into consideration in operations such as pulp washing. The ion exchange capacity of the high-yield pulps is higher than that of chemical pulps, particularly because they contain much lignin carry­ ing sulfonic acid groups.

7.4 Special Features of High-Yield Pumping

163

TABLE 7-11. Type of Acidic Groups in Wood3

Structure0

Acidic group Carboxylic

pKa (25°C)

Degree of ionization at pH 7 (%)

R-C0 2 H (minor)

4-5

99-99.9

R-CH(OR')C02H

3-4

99.9-99.99

7-8

10-50

0 Phenolic

R - C ^ ^ - O H (minor)

R-^3-0H

Alcoholic

R-CH(0H)-R' (minor) R-CH(0R')CH(0H)-R"

Hemiacetalic

ch <

-0 )-0H

9.5-10.5

15-17 13.5-15

12-12.5

0.03-0.3

—ft

10

— fi

-10

10~6-3·1θ"5

-3 -4 10 - 3 - 1 0

OR

a b

From Sjöström (1989). R, R', R" = H, alky!, or aryl.

The acidic groups favorably affect the fiber properties. The polyelectrolytic and colloidal properties of the pulp, regulating the extent of swelling as well as the brightness stability, are more or less, and often even markedly, influenced by these charges. The acidic groups are indeed important factors contributing to the hydrodynamic properties of the fiber suspensions as well as to the physical properties of fibers and forces between them. Various ionizable groups are present in the polymeric constituents of wood (Table 7-11). Of these, however, only the carboxyl groups are ionized in neutral or weakly acidic conditions. Ionization of phenolic hydroxyl groups demands rather alkaline conditions, and the very weak alcoholic hydroxyl groups are ionized only in the presence of strong alkali. However, due to the inductive effect of certain substituents that are present in wood constituents, the acidity of the hydroxyl groups can increase considerably.

164

Chapter 7. Wood Pulping

TABLE 7-12.

Carboxyl Groups in Some Wood Species (mmol/100 g) a Carboxyl group content Accessible to ion exchange Species

Picea abies Pi nus sylvestris Betula verrucosa/pubescens a b

Methylglucuronic acid content

Total

Untreated

Hydrolyzed b

7 8 15

15-25 15 25-35

7 5 6

13 9 17

From Sjöström (1989). The sample has been treated to hydrolyze the carboxylic acid lactones and esters.

At usual conditions of papermaking the carboxyl groups are obviously the only type of functional groups giving rise to the generation of charged sites on the fibers. Both in native wood and in mechanical pulps the majority of the carboxyl groups are of uronic acid type mainly attached to the xylan. Some of them originate from pectic substances, enriched in the compound middle lamella region. Native lignin contains few, if any, carboxyl groups. An additional source of the carboxyl groups is the fraction of free fatty acids and resin acids, but their contribution is relatively small. Some of the carboxyl groups are located at inaccessible regions in the wood cell wall or they are blocked through esterification or lactonization. These groups are liberated during alkaline pretreatment (Table 7-12). As a result, the solvation and swelling of the material are increased, leading to improved bonding of the fibers. When the pretreatment is made in the presence of sulfite, as usual, lignin is sulfonated to a degree depending on the treatment conditions. The strong­ ly acidic sulfonic acid groups introduced are ionized even at a low pH when they are in a free hydrogen ion form. However, their contribution to swelling and other material properties depends also on the particular counter ion. Whether or not it is possible to improve the properties of high-yield pulps by more proper treatments leading to a smooth liberation of the fibers and by increasing the concentration of acidic material on the fiber surfaces are open questions.